The present invention relates to improved frequency compensation for analog integrated circuits, especially integrated circuit devices used in implementing voltage regulator circuits and other negative feedback circuits requiring ripple rejection.
As is well known, the basic architecture of a series-type voltage regulator circuit includes a pass element (e.g., a transistor) connected in series between a load and an unregulated supply line, an error amplifier which controls the pass element, a voltage reference generator connected to one input of the error amplifier and a negative feedback network connected between the other input of the error amplifier and the output of the regulator circuit. This type of architecture has been incorporated in many commercially available integrated circuits.
Most applications of a voltage regulator circuit require the circuit to be stable. Like any other circuit operating with negative feedback, a series-type voltage regulator circuit will tend to become unstable (i.e. break into oscillations) if total phase shift across the circuit for a particular frequency approaches 180.degree. and, simultaneously, the magnitude of the gain of the circuit at that frequency approaches unity. In an idealized voltage regulator circuit, stability is not a problem because total phase shift is limited to 90.degree. and overall gain exhibits a single-pole rolloff characteristic corresponding to a -6 dB/octave rolloff of dc open-loop gain for frequencies in excess of a relatively low 3-dB bandwidth w.sub.0.
However, as a result of excess phase shifts associated with active devices and stray capacitances inherent in practical voltage regulator circuits, total phase shift across such circuits typically exceeds 180.degree. at frequencies well within a desired range of operation. Therefore, it is usually necessary to compensate practical regulator circuits to provide stable operation. Such compensation is typically required in operational amplifier circuits as well.
Commonly, capacitors are placed in voltage regulator and operational amplifier circuits to provide frequency compensation. However, the integration of a voltage regulator or operational amplifier circuit complicates such frequency compensation by limiting the points that an external capacitor can be connected, and by imposing size constraints on internal capacitors.
One known frequency compensation technique associated with integrated circuit voltage regulator devices is to use an internal capacitor to reduce the gain of the differential input amplifier stage of the regulator at high frequencies. This technique is used, for example, in the LM120 integrated circuit negative voltage regulator device available commercially from various manufacturers. The differential input amplifier stage of the LM120 includes a differential amplifier coupled to a current mirror load. The current mirror load acts as a gain stage, and performs differential-to-single-ended signal conversion. The gain of the differential input amplifier stage is rolled off by coupling a bypass capacitor across the base-collector junction of the output transistor of the current mirror load. This approach, however, has shortcomings and disadvantages that limit the operating characteristics of the circuit.
For example, even with the internal capacitor, a differential input amplifier stage of the type in the LM120, which has npn-pnp composite transistors at its inputs, exhibits a gain of at least 0.25, no matter what the value of the internal capacitor is. In standard circuits that have a single transistor at each input instead of a composite like the LM120, this minimum gain is typically 0.5. Because the output stage of the device also introduces gain into the regulator system loop, unstable circuit conditions may still result. Also, regulator circuits, especially when used in power supplies, typically include an external capacitor coupled across the load to absorb transients. The external capacitor introduces a pole into the regulator system loop that, when combined with the pole created by the internal capacitor, can cause excess phase shift tending to exacerbate stability problems.
In most operational amplifier circuits, the voltage gain of the input stage can be rolled-off by placing an internal or external capacitor across the output stage, thus providing simple frequency compensation. However, the capacitor tends to pass signals at the amplifier supply line directly to the output. In a regulator circuit having a rectified ac voltage signal at its input, the capacitor would create ripple in the output voltage. This compensation technique therefore is not suitable for regulator circuits, or even for some operational amplifier circuits, where it is desired that the circuit have an ability to reject fluctuations of voltage signals on the power supply lines (i.e. ripple rejection).
It would therefore be desirable to be able to provide a differential input amplifier stage for a voltage regulator or other circuit that does not require a large capacitor for frequency compensation, and that therefore can be implemented as part of an integrated circuit. It would also be desirable to be able to provide a differential input amplifier stage that can be compensated without degrading ripple rejection. Further, it would be desirable to be able to provide a differential input amplifier stage having a gain that can be rolled substantially below 0.25 (for a composite npn-pnp type input) or 0.5 (for a single transistor type input) over some range of frequencies and that can be compensated without causing stability problems when load conditions introduce an external pole approaching a 90.degree. phase shift.